1. Field of the Invention
The present invention relates to an image display apparatus and, more particularly, to the image display apparatus, for example, which is constructed using a liquid crystal display element as a display means for displaying an image and constructed to permit an observer to observe an enlarged image of image information displayed on the display element through a display optical system having a free surface prism properly set and which is suitably applicable to head-mounted displays (which will be referred to hereinafter as xe2x80x9cHMDsxe2x80x9d), goggle type displays, and so on.
2. Related Background Art
A variety of proposals have been made heretofore about the image display apparatus of the head-mounted type, so called the head-mounted displays, which permitted the observer to observe an enlarged virtual image of the image information displayed on the image display element of liquid crystal or the like, for example, using an optical element provided with an entrance surface, a plurality of reflecting surfaces, and an exit surface in surfaces of a transparent body (prism).
As the image display apparatus of this head-mounted display type, for example, configurations of optical systems, particularly, excellent in terms of size reduction are proposed in Japanese Patent Application Laid-Open Nos. 7-333551, 8-179238, 8-234137, and so on.
On the other hand, the HMDs with wider angles of view using a prism having surfaces with refractive powers differing depending upon azimuthal angles (such surfaces will be referred to hereinafter as free curved surfaces and the prism as a free surface prism), are proposed, for example, in Japanese Patent Application Laid-Open Nos. 10-75407, 9-146037, (10-282421,) and so on. In the HMDs proposed in these applications, the display optical system is constructed in such structure that a lens separate from the free surface prism is interposed between the eyes and the free surface prism, thereby achieving the wide horizontal angle of view of about 60xc2x0.
In the field of the image display apparatus such as the HMDs or the like, an important issue is to decrease the size and weight of the entire apparatus, because the apparatus is mounted on the observer""s head. Another important issue is to allow the observer to observe the image information displayed on the display means, with a wide angle of view and in good order.
The display optical system suggested in Japanese Patent Application Laid-Open No. 10-75407 includes a lens between the eyes and the free surface prism. This increases the size of the display optical system. In order to form a seethrough optical system for permitting observation of the outside, three prisms are necessary for establishing a nearly non-power seethrough optical system while achieving approximate agreement between the eye optical axis and the outside optical axis (FIG. 15 of Laid-Open No. 10-75407), and thus the entire optical system tends to become great.
As apparent from the path diagram, distortion of the display optical system tends to become great.
An object of the present invention is to provide an image display apparatus that permits establishment of a compact display optical system and good correction for distortion and various aberrations and that is optimal for the HMDs with wide angles of view.
Another object of the present invention is to provide an image display apparatus that permits observation of image information with a wide angle of view and with good image quality while decreasing the size of the entire apparatus and loss of light amount, by properly setting the structure of the display optical system including a free surface prism body for guiding light from the display means to the observer""s eyes, in order to permit the observer to observe the image information displayed on the display means such as a liquid crystal display or the like.
An image display apparatus according to one aspect of the present invention is an image display apparatus comprising display means for displaying image information, and optical means for guiding light from the display means to the eye, wherein said optical means comprises a prism body of a positive refractive power having three or more surfaces with refractive powers differing depending upon azimuthal angles, and wherein a surface of the prism body closest to the display means is a surface in which refractive powers thereof differ depending upon azimuthal angles and in which a local curvature radius area having a positive refractive power on a local generating-line section (meridional section) is interposed between local curvature radius areas having a negative refractive power on the local generating-line section (meridional section).
In a further aspect of the present invention, said prism body comprises a transmitting surface, a reflecting surface, and a reflecting surface in the order named in the opposite direction to passage of light, from the surface closest to the eye, and refractive powers on a local sagittal section at hit points between the respective surfaces of the prism body and a reference ray are arranged in such arrangement that the transmitting surface of a negative refractive power, the reflecting surface of a positive refractive power, and the reflecting surface of a negative refractive power are arranged in the order named in the opposite direction to passage of light, from the eye side.
In a further aspect of the present invention, the following condition is satisfied by a refractive index Nd of a material of said prism body:
1.54 less than Nd less than 1.85.
In a further aspect of the present invention, the following condition is satisfied by an angle a between the reference ray and a tangent line to the surface is closest to the eye on the local generating-line section (meridional section) at the hit point between the the reference ray and the surface of said prism body closest to the eye;
70xc2x0 less than xcex1 less than 95xc2x0.
In a further aspect of the present invention, said prism body comprises a transmitting surface, a reflecting surface, and a reflecting surface arranged in the order stated from the surface closest to said display means, and refractive powers on the local generating-line section (meridional section) at a hit point on each surface of the prism body, of a ray passing the center of the eye and a most marginal image on the far side from the eye out of most marginal images on the local generating-line section (meridional section) of the display means, are arranged in such arrangement that the transmitting surface of a negative refractive power, the reflecting surface of a positive refractive power, and the reflecting surface of a positive refractive power are placed in the order stated from the display means side.
In a further aspect of the present invention, the surface closest to the eye is a surface having two functions of transmitting action and reflecting action.
An image display apparatus according to one aspect an of the present invention is an image display apparatus comprising display means for displaying image information, optical means for guiding light from the display means to the eye, and seethrough means for guiding light from the outside to the eye, wherein said optical means comprises a prism body of a positive refractive power comprised of three or more different surfaces, inclusive of two or more surfaces with refractive powers differing depending upon azimuthal angles, wherein said seethrough means is constructed in such structure that a correction prism is placed adjacent to or joined to the prism body in said optical means, that adjacent surfaces or joint surfaces are partially transmitting surfaces with little optical refractive power, and that the eye optical axis is approximately aligned with the outside optical axis, wherein a surface of the seethrough means closest to the outside is a decentered, curved surface which has a shape different from that of a surface of the seethrough means closest to the eye and in which refractive powers differ depending upon azimuthal angles, and wherein the following condition is satisfied by a curvature radius l_ry on a local generating-line section (meridional section) and a curvature radius 1_rx on a local sagittal section at a hit point between a reference ray and the decentered, curved surface of the seethrough means closest to the outside:
|l_rx| less than |l_ry|xc2x7
In a further aspect of the present invention, said correction prism is placed in an outside region of said prism body.